I recently heard about Magicka and I really want to play it. I told my friend about it, and so does he, but we don't really want to buy it twice (we would prefer to pay for one copy and just play from that). I know that I can download a game that is in my Steam library, and I wanted to know if he could play it on another computer if I were to buy it.
As this question reveals you can kinda sorta do that if you make an ad-hoc account and then take turns with logging in with it, nevermind you're breaking the Steam Terms of Service, but who cares about that, eh? ;)
Even then, the two of you can't play at the same time with just one copy. If you want to play with your buddy, you'll have to buy two copies.
If the price is too steep, practice patience. Steam deals can be amazingly good.
A really roundabout workaround that doesn't break ToS is to use RDC or some sort of input/output streaming platform, setup a virtual controller and map some keyboard keys to it, allowing the two of you to play magicka from the same computer remotely.
You will probably run into issues attempting this, and the end result will probably be less than satisfactory. Best way is to just buy the game.
other alternative is either get a controller for your friend or setup a virtual controller that responds to keyboard controls. which will allow you two to both play from the same PC.